Many refineries devote extraordinary amounts of energy and operating expense to convert most of a whole crude oil feed into high octane gasoline. The crude is fractionated to produce a virgin naphtha fraction which is usually reformed, and a gas oil and/or vacuum gas oil fraction which is catalytically cracked to produce naphtha, and light olefins. The naphtha is added to the refiners gasoline blending pool, while the light olefins are converted, usually by HF or sulfuric acid alkylation, into gasoline boiling range material which is then added to the gasoline blending pool.
The fluid catalytic cracking (FCC) process is the preferred process in the petroleum refining industry for converting higher boiling petroleum fractions into lower boiling products, especially gasoline. In FCC, a finely divided solid cracking catalyst promotes cracking reactions. The catalyst is in a finely divided form, typically with particles of 20-100 microns, with an average of about 60-75 microns. The catalyst acts like a fluid (hence the designation FCC) and circulates in a closed cycle between a cracking zone and a separate regeneration zone.
Fresh feed contacts hot catalyst from the regenerator in the base of a riser reactor. There are many localized catalyst currents and eddies, and many refiners now start the catalyst flowing smoothly up the riser by injecting some sort of lift gas. The cracked products are discharged from the riser, separated, and cracking reactor sent to a main fractionator which produces several product streams.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking With Zeolite Catalysts", Venuto and Habib, Marcel Dekker, N.Y., 1978, incorporated by reference.
One of the most complex, and least understood parts of the FCC process is where catalyst contacts feed in the base of the riser. Much effort has been spent developing better FCC nozzles to achieve a more uniform spray pattern into the riser), riser base designs to promote catalyst mixing, or multi-nozzle control schemes.
U.S. Pat. No. 5,108,583 is one of many FCC feed nozzle patents, and is incorporated herein by reference.
U.S. Pat. Nos. 4,717,467 and 4,578,183 which are incorporated by reference, are directed to modifying the riser base to make better use of feed nozzles. A venturi section, or draft tube is used to promote better contact of feed and hot catalyst.
These approaches, better nozzles, different riser configurations, have never been as successful as desired, due in part to the nature of the FCC process. Change is constant in FCC, due to changes in feed rate and type, and catalyst circulation. Feed nozzles plug, valves erode, and flow patterns in the base of the riser may change constantly.
U.S. Pat. Nos. 4,808,383, Buyan, incorporated by reference, recognized the difficulty of achieving the perfect nozzle, or the perfect riser base shape for perfect contacting, and resorted to individual control of multiple nozzles, with the nozzle flow control valves driven by taking a temperature profile across the riser.
While such an approach works, it requires the insertion of a probe through a packing gland in a purge nozzle in the riser. Because of harsh conditions in the riser it is not practical to leave thermocouples permanently installed in the riser, and not safe to make frequent temperature profiles through packing glands providing access to the riser. It was best suited to a one time probe, of a line across one elevation of a riser.
I wanted an approach with more depth to it. I wanted the benefits of the '383 approach to improving catalyst:oil mixing in the base of a riser, but without the '383 limitations.
I realized that it was possible to use a non-intrusive method of measuring the riser density in a plane across the riser, at one (or even multiple) elevations in the riser. I developed a way to make continuous, non-intrusive, measurements of density in the riser, and use this information to control directly the flow of feed to individual nozzles.